The correct option is C thoughtless
The word 'calculated' refers to something done with full awareness of the likely consequences.
Option A, idiotic refers to being very stupid.
Option B, simple refers to something easily understood and done.
Option C, thoughtless refers to not showing consideration for the needs of other people.
Option D, artless refers to being without guile or deception.
Thus, according to the meaning, the word 'thoughtless' is the opposite of the word 'calculated'.